The present invention relates generally to position referencing devices and more particularly relates to techniques for correcting systematic errors found in commercial position encoders and for accurately extrapolating encoders to higher resolution.
A wide range of devices across a wide range of applications require accurately tracking and referencing the position of a moving component. One method of tracking the position and motion of a controlled component is with the use of a quadrature position encoder responsive to an strip encoded with a regular periodic pattern. The use of an encoded strip and quadrature position encoder combination provides a relatively simple yet accurate tracking device for referencing the position of the moving component. However, it has been observed that such commercial quadrature position encoders typically possess and exhibit a systematic error pattern. This systematic error pattern, although within the specifications of the encoder, can generate adverse results.
Consider, for example, liquid ink printers, such as piezoelectric, thermal, acoustic or phase change wax-based, in which droplets of ink are ejected from a print head towards a recording sheet. In a carriage type printer, the print head is attached to a carriage which is reciprocated to print one swath of information on a stationary recording medium, such as paper or a transparency. As the print head is passed across the recording medium ink drops are expelled from the print head at the appropriate time (position across the medium) to generate an accurate printed image. After the swath is printed, the paper can be stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed.
Reciprocating carriage printers typically include a position encoder, such as a quadrature encoder or similar device, sensing the pattern on an encoder strip (also referred to as a fence) which includes a series of fiducial marks such as alternating black and clear bars. The use of a fence and quadrature position encoder combination operates as a tracking device for referencing the position of the print head to enable firing the print head with the correct timing. However, as mentioned above, commercial quadrature position encoders typically possess and exhibit a systematic error pattern which can generate objectionable results in the printed image.
This error pattern is illustrated in the graph of FIG. 1, wherein there is plotted a curve showing the time between transitions for a commercial quadrature encoder used to reference the position of a print head as the carriage scans the recording medium at a constant speed. The data was collected using a commercial quadrature encoder with a fence pattern that was repeated 150 times per inch, providing a resolution of 600 transitions per inch. As can be seen in FIG. 1, the encoder possess a systematic error wherein the timing for every fourth transition is significantly different from the others. Using this configuration to print at 600 spots per inch results in every fourth pixel being laterally displaced by one quarter of a pixel. This systematic drop placement error is one cause of image defects and print quality degradation.
In accordance with one aspect of the present invention, there is provided a system for referencing the position of a moving component. The system includes an encoder strip having a repeating pattern of fiducial marks; a first sensor, the first sensor and encoder strip being positioned to move relative to each other at a speed that is proportional to the speed the moving component, the first sensor generating a first signal in response to the relative movement of the first sensor and encoder strip, the first signal including a plurality of transitions corresponding to the repeating pattern; a second sensor, the second sensor and encoder strip being positioned to move relative to each other at a speed that is proportional to the speed the moving component, the second sensor generating a second signal in response to the relative movement of the second sensor and encoder strip, the second signal including a plurality of transitions corresponding to the repeating pattern; and a controller connected to receive the first signal and the second signal, the controller being responsive to the first and second signals to identify a direction of movement of the moving component, the controller being further responsive to the first signal to estimate a position of the moving component.
Pursuant to another aspect of the present invention, there is provided a method of referencing the position of a moving component. The method includes generating a first signal in response to the relative movement between a first sensor and encoder strip having a repeating pattern of fiducial marks, the first signal including a plurality of transitions corresponding to the repeating pattern; generating a second signal in response to the relative movement of a second sensor and the encoder strip, the second signal including a plurality of transitions corresponding to the repeating pattern; estimating a position of the moving component based upon the transitions within the first sensor signal; and identifying a direction of movement of the moving component in response to the first and second signals.